Frequency filters produced using thin-film technology, for example, BAW filters (bulk acoustic wave filters) or specific SAW filters (surface acoustic wave filters), find application as a frequency-determining component in transmitters and receivers which operate in the frequency range of several hundred MHz to approximately 20 GHz. Such filters are used in radio-frequency technology, for example, in mobile radio and WLAN.
SAW filters and BAW filters are passive filters having usually a bandpass filter characteristic wherein an acoustic wave is generated from an electrical signal and, vice versa, with the aid of the piezo-effect. In a SAW filter, the acoustic wave propagates at the surface of the piezo-substrate, while the acoustic waves propagate through the piezo-substrate in the BAW filter.
On account of their particularly advantageous electrical and physical properties, BAW filters are increasingly replacing SAW filters in radio-frequency technology. BAW filters are available for pass frequencies of approximately 1 GHz to 20 GHz, have a lower insertion loss (0.5 dB) than SAW filters and achieve a quality factor of more than 1000. At the same time, BAW filters can be realized in smaller structural sizes and are generally more cost-effective to produce.
During production of BAW resonators using thin-film technology, the piezo-electric thin-film layer, for example, an aluminum nitride, zinc oxide or PZT layer is deposited on a carrier using reactive sputtering deposition. The quality of the piezo-layer crucially influences the technical properties of the BAW resonator. A crystalline and highly oriented piezo-layer is particularly advantageous and places stringent demands on the deposition method and the deposition conditions such as, for example, pressure, temperature, homogeneity of the substrate and purity of the media.
The piezo-layer is usually grown heteroepitaxially above a metal and carrier layer in BAW manufacture. This form of layer growth leads to a columnar, polycrystalline piezo-layer. One disadvantage of that type of layer growth is that growth defects in the crystal microstructure occur along topology edges on the carrier layer. Such growth defects have consequences for the technical properties and reliability of the BAW resonators.
The problem is explained below by way of example with reference to FIG. 1. FIG. 1 shows, with the aid of a micrograph recorded by a scanning electron microscope, an excerpt from a BAW resonator in cross section during production. A multilayer electrode comprising a lower first, corrosion-sensitive metal layer (M1) and an upper second metal layer (M2) is arranged on a carrier layer (TS) composed of silicon dioxide. The transition from the upper metal layer to the lower metal layer is characterized by a flat topology edge, and the transition from the lower metal layer to the carrier layer is characterized by a steep topology edge. A piezo-layer (PS) is deposited above the metal layers and the carrier layer. During the growth of the piezo-layer, the flat topology edge at the transition from the upper metal layer to the lower metal layer has led to a slight disturbance in the crystal microstructure (imperfection A). The imperfection A is characterized by disturbed grain growth. This is a slight disturbance since the two piezo-layer regions on the left and right of the imperfection are in good contact. The steep topology edge at the transition from the lower metal layer to the carrier layer has led to the formation of a great disturbance in the crystal microstructure (imperfection B), which results in a gap or even cavity extending through the entire piezo-layer. Along such growth defects such as are evident from FIG. 1, liquids can penetrate during subsequent wet-chemical processes and corrode corrosion-sensitive metal layers. This occurs, in particular, if aluminum, titanium, titanium nitride, silver or copper or multilayer systems comprising these materials are used as metal layer material.
Corrosion of the metal layers leads to poor electrical properties of the BAW resonators and constitutes a quality risk that is difficult to calculate with regard to the reliability of these components. A further problem is that during the subsequent deposition of a metallic upper layer for the counterelectrode on the piezo-layer, a metallic extension can form in the cavity and, in the worst case, leads to a short circuit in the electrode.
To improve the technical properties and avoid corrosion in the bottom electrode of BAW resonators, it is known that the bottom electrode can be embedded into a dielectric with the aid of CMP processes (U.S. Pat. No. 7,657,983). This method is very complex in terms of process engineering, however, since the dielectric deposited over the whole area has to be removed from the surface of the electrode by chemical mechanical polishing to yield a planar surface comprising electrode and dielectric, on which planar surface the piezo-layer can grow.
It is also known to smooth steep topology edges by chemically etching the metal layers. For aluminum, in particular, no reproducible etching processes that lead to oblique sidewalls exist here, however.
A further alternative is to avoid corrosion-sensitive constituents in the bottom electrode. However, non-corrosion-sensitive metals generally have a poor conductivity. This necessitates very thick electrode layers and is therefore suitable only for specific designs of BAW resonators. A likewise unsatisfactory solution is to avoid corrosive processes during the manufacture of BAW resonators, which necessitates new process steps and yields uncertain results.
It could therefore be helpful to provide a structure suitable for BAW electrodes, for example, together with a production method with which the problems that occur during layer growth can be avoided and, in particular, susceptibility of the lower metal layers or electrodes to corrosion can be reduced.